Crystal chemistry of three tourmalines bby SREF, EMPA, and SIMS

The crystal structures of three tourmaline crystals: (Na0.49 K0.01 Ca0.48) (Mg1.35 Fe2+0.94 Fe3+0.49 Ti0.20) (Al4.58 Fe3+0.62 Mg0.80) (Si5.99 Al0.01) O18 (BO3)3.03 (OH)3.18 F0.18 O0.64, a = 16.017(2), c = 7.256(2) Å, V = 1612.2(4) Å3, R3m, Z = 3; (Na0.64 K0.01 Ca0.03) (Mn0.18 Fe2+1.71 Al0.88 Li0.11 Zn0.03 Ti0.07) (Al5.67 Fe3+0.28 Mg0.05) (Si5.76 Al0.24) O18 (BO3)2.99 (OH)3.96 F0.17, a = 15.983(2), c = 7.152(2) Å, V = 1582.1(4) Å3; (Na0.81 K0.01 Ca0.01) (Mn0.02 Mg0.61 Fe2+0.90 Al0.80 Li0.70 Zn0.01 Ti0.06) Al6.00 (Si5.97 Al0.03) O18 (BO3)2.93 (OH)3.42 F0.55 O0..03, a = 15.921(3), c = 7.137(2) Å, V = 1566.7(6) Å3, have been refined to R-indices of 1.3–2.2% using X-ray intensity data collected with a four-circle diffractometer using MoKα X-radiation. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe3+/(Fe2+ + Fe3+) ratio was calculated for electroneutrality. The refined site-scattering values and the observed <Y-O> and <Z-O> distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O2− is dominant at the W(O1) site, and is an “oxy” tourmaline as defined by Hawthorne and Henry (1999). Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site.